Heat transfer labels are used to decorate items by effecting transfer of the label to the item upon application of heat and pressure. The labels consist of inks containing a thermoplastic which softens and flows when heated so that it conforms to the decorated surface. Currently, known thermoplastic ink systems are solvent or water-based type systems, with solvent-based systems predominating.
Ultraviolet-curing ink systems which cure upon exposure to ultraviolet light are generally known in the art. NOR-COTE INTERNATIONAL provides UV-curing inks for the screen printing industry known as Series 02, Series 70, and Series 80. These inks, however, have not performed adequately when applied to webs used to apply heat transfer labels. With application of heat and pressure, these labels did not transfer to styrene or acrylonitrile-butadiene-styrene (ABS) copolymer, which are the most readily decorated substrates, regardless of the variance of temperature, pressure, and time. Under-curing the ink provided transfer of some of the ink to the substrate as a ghost-image, but it does not offer any adhesion to the part. Simply affixing tape over the label and removing the tape resulted in the removal of the label as well.
More recently, ink manufacturers, including Craig Adhesives & Coatings Company, of Newark, N.J. have developed coatings that will accept writing, glue or hot stamp decoration while maintaining good intercoat adhesion. However, when tested for use as heat transfer inks, new coatings did not provide enough thermoplasticity and tack for transfer and adhesion to ABS, styrene or other common substrates.
For an ink to be effective in the preparation of a heat transfer label, it must soften and develop tack at a temperature above room temperature and possess sufficient cohesive strength to transfer completely from the carrier web to the substrate to be decorated. Once adhesion has been established on the substrate, it must be strong enough to retain the complete image as the carrier web is removed from the ink and substrate. Once the decorated part has cooled to room temperature, the ink must pass minimal adhesion tests, normally specified in the industry by applying an adhesive tape to the ink surface and pulling the tape away from the substrate at right angles to the surface in an attempt to remove the image with the tape.